Band filter circuits



March 28, 1939. B. D. H. TELLEGEN BAND FILTER CIRCUITS Filed A ril 22. 1935 N Ra Y 0 E Tu N NE R wT/ m N g T l D A Patented Mar. 28, 1939 purse STATES lands, assignor to Radio Corporation of America, a corporation of Delaware Application April 22, 1935, Serial No. 17,620 In Germany August 13, 1934 7 Claims.

This invention relates to a band filter constituted by capacitatively coupled oscillatory circuits and has for its object to improve the properties of such a band filter.

In order that the invention may be clearly understood and readily carried into effect reference is made to the accompanying drawing wherein Figures 1 and 2 show known types of band pass filter circuits for the purpose of explaining the present invention, and Figures 3 and 4 show two embodiments of band pass circuits according to the invention.

Figure 1 shows a commonly known circuit arrangement of a band filter constituted by two similar oscillatory circuits L1 C1 and L2 C2 capacitatively coupled by means of a condenser CK. As is well known, such a system has in the case of a sufficiently tight coupling two resonance frequencies lying symmetrically with respect to the resonance frequency W of the oscillatory circuits. If in addition it is assumed that the two oscillatory circuits are similar, a perfectly symmetric resonance curve is obtained for the circuit set up by the coupling.

In this case, the coupling of the oscillatory circuits was assumed to be purely capacitative. In practice, however, this assumption is generally not correct, as the coupling condenser CK has inherent to it such losses that a resistance is to be assumed in parallel or in series with CK. In amplifying circuit arrangements in which such a band filter is included in the grid circuit of an amplifier valve the coupling condenser CK may, for example, be shunted by a resistance for thepurpose of applying a suitable grid bias to the amplifier valve. In all those cases in which a resistance is connected in parallel or in series with the coupling condenser, the coupling factor is no real positive value and this results inthe resonance curve of the system becoming dissymmetrical.

In the circuit arrangement showniin Figure 1 the coupling condenser is arranged in such man- 'ner that the coupling becomes looser when the frequency to which, the band filter is tuned increases. The, same difliculties as in the circuit arrangement shown in Figure 1 are encountered when the coupling condenser shown in Figure 2 is arranged in such manner that the coupling becomes tighter with increasing frequency.

Now, the present invention permits of obtaining a symmetrical resonance curve in such a circuit arrangement of capacitatively coupled oscillatory circuits independently of the resistance connected in series or parallel with the coupling condenser.

According to the invention this is ensured by coupling to each other two oscillatory circuits by means of one or more circuits comprising two 5 condensers and one resistance which are starand/or delta-connected.

Figure 3 shows a band filter according to the invention. The two inductance coils L1 and L2 and the tuning condensers C1 and C2 have connected in series with them condensers 0K1 and 0K2. The points of connection between the latter and the coils L1 and L2 are interconnected by an ohmic resistance R and the points of connection to the tuning condensers are interconnected directly. As found by calculation, although the condensers CK1 and 0K2 introduce losses, the coupling factor has a real positive value and a symmetrical resonance curve can be obtained.

The same result is obtained when-the two oscillatory circuits shown in Figure 4 are interconnected on the one hand directly and on the other hand through the series connection of two condensers CH1 and 0K2 and the point of connection between the two coupling condensers is connected by an ohmic resistance R to the direct connection of the oscillatory circuits.

The difierence between the two circuit arrangements shown in Figures 3 and 4 is that with increasing frequency a decreasing coupling is obtained in the first and an increasing coupling in the latter arrangement. The two circuit arrangements may be used at the same time when coupling varying in a given manner with the frequency is to be obtained.

It is well known that symmetrical band pass characteristics are obtainable from circuits such as Fig. 1 and Fig. 2 when the two component circuits are coupled by a pure reactance. It is also well known that dissymmetry appears when the coupling impedance is not a pure reactance, or else as heretofore stated when the coupling factor is no real positive value.

In some cases it may not be feasible for economic reasons to provide a truly low loss condenser for the coupling impedance for example of Fig. 1, especially if the required capacity is large.

In this case the coupling impedance may be considered as a pure reactance in series with a certain efiective resistance.

According to the invention a coupling net-work may be made up of two imperfect condensers 0K1 and CKz together with a resistance as shown in Fig. 3, and if this resistance is suitably proportioned the coupling action of this network very closely approximates the action of a single ideal condenser connected as in Fig. 1.

To find suitable values for the coupling network the following procedure may be used: The voltage introduced into either of the tuned cir cuits by unit current in the other circuit should be 90 degrees out of phase with this unit current in order that the coupling network act like a perfect condenser. This voltage is easily written down in terms of the circuit constants of the R, CKI, CKz network and it will be found that the real portion of this voltage substantially vanishes when the resistance R across the tops of the two condensers CK1, CKz (Fig. 3) is made equal to twice the effective resistance in series with each of the condensers (this is assuming the two condensers are alike). Thus the voltage under these conditions introduced into one circuit by unit real current in the other circuit is purely imaginary, and hence the entire structure behaves in a desired fashion corresponding to the way Fig. 1 would behave without any losses in the coupling condenser.

Since the invention is directed toward an arrangement that has the effect merely of neutralizing losses in a coupling condenser, it may be used whether the two circuits of Fig. 1 are tuned exactly alike or not. That is to say, the circuit of Fig. 1 may be employed even without identical tuning of the two circuits, and the present invention does not depend for its operation or usefulness upon the state of tuning of these two circuits. They were mentioned as being similar circuits only because that is the more common adjustment.

In other words, the invention does not in itself provide band pass characteristics. It merely removes a certain defect from well known circuits so as to permit them to provide the band pass characteristics for which they were designed. For this reason it is not thought necessary to demonstrate the existence of a symmetrical resonance curve for the invention since it is well known that symmetrical curves are obtained from filters with pure reactance coupling, and it has already been demonstrated that the invention provides effectively pure reactance coupling in spite of the use of imperfect coupling condensers.

What I claim is:

1. A band pass filter comprising a pair of oscillatory circuits, means to capacitatively couple said oscillatory circuits, the magnetic coupling between said circuits being substantially zero, said coupling means comprising two condensers connected together directly and in series and a resistance associated therewith.

2. A circuit arrangement comprising two oscillatory circuits having coupling means between them comprising a resistance and a pair of condensers which are interconnected in star fashion, said pair of condensers being connected together directly and in series.

3. A circuit arrangement comprising two tuned circuits, each including an inductance, a variable condenser and a fixed condenser connected in series, a conductive connection between corresponding terminals of the fixed condensers, and a resistance connected between the other terminals of said condensers.

4. A circuit arrangement comprising two similarly tuned circuits, each including a series-connected inductance and fixed condenser which are shunted by a variable condenser, a resistance connected between the points of connection of eachinductance and its fixed condenser, and a conductive connection between the points of connection of each variable condenser and its fixed condenser.

5. A circuit arrangement having band pass characteristics comprising two oscillatory tunable circuits, each including an inductance and a condenser in parallel, one pair of corresponding terminals of said tunable circuits having connected between them a pair of series connected coupling condensers, the other pair of corresponding terminals of the tunable circuits having a common connection, and a resistance connected between said series connected coupling condensers and the common connection.

6. A circuit according to claim 5 wherein said oscillatory circuits include variable condensers which are similarly tunable by means of common operating means.

7. A band pass filter comprising a pair of oscillatory circuits, means for providing capacitive coupling only between said circuits comprising a pair of condensers and a resistance which have a terminal in common, the other terminal of each of said condensers being connected respectively to one of the oscillatory circuits and the other terminal of said resistance being connected to both oscillatory circuits.

BERNARDUS D. H. TELLEGEN. 

